1. Field of the Invention
The present invention relates to a bonding apparatus and more particularly to a bonding apparatus that calculates the amount of offset between a bonding tool and a position detection camera that takes images of a workpiece.
2. Prior Art
Manufacturing process of semiconductor assemblies such as ICs, etc. includes a wire bonding steps. In such bonding steps, a wire 4 is, as shown in FIG. 7, connected to a pad (a first bonding point) 1a of the semiconductor chip 1 of a workpiece 3 and a lead (second bonding point) 2a of a lead frame 2.
The above wire bonding steps are performed by, for instance, a wire bonding apparatus 10 shown in FIG. 8.
First, a deviation from normal positions of at least two fixed points on a semiconductor chip 1 and at least two fixed points on a lead frame 2 is detected by a position detection camera 11, and bonding coordinates stored in memory (not shown) beforehand are corrected based upon such detected values. When this detection is done by the position detection camera 11, an X-axis motor 12 and a Y-axis motor 13 are first driven so that the optical axis 11a of the position detection camera 11 is positioned directly above a measurement point. After the bonding coordinates are corrected as described above, a bonding tool (called xe2x80x9ctoolxe2x80x9d) 5 is moved in the directions of the X and Y axes (or moved horizontally) and in the direction of the Z axis (or moved vertically), and a wire 4 that passes through the tool 5 is wire-bonded to the first bonding point 1a and the second bonding point 2a. 
In the above structure, the optical axis 1a of the position detection camera 11 and the axial center 5a of the tool 5 are offset by a distance W. Accordingly, after the deviation of the fixed points is detected by the position detection camera 11 and the bonding coordinates are corrected, the XY table 15 is moved by an amount equal to the offset amount W by the X-axis motor 12 and Y-axis motor 13, thus positioning the tool 5 above the first bonding point 1a. Afterward, the wire 4 is wire-bonded at the above-described corrected bonding coordinates by the movement of the XY table 15 in the directions of the X and Y axes by the X-axis motor 12 and Y-axis motor 13 and by the movement of the tool 5 in the direction of the Z axis by the vertical movement (or swinging movement) of the bonding arm 16 caused by the Z-axis motor 14.
In FIG. 8, the bonding arm 16 is disposed on a bonding head 17 so that the bonding arm 16 is free to swing, and the position detection camera 11 is fastened to the bonding head 17 via a camera holding arm 18. Xw refers to an X-axis component of the offset amount W, and Yw refers to a Y-axis component of the offset amount W.
The position detection camera 11 is for determining the reference points for ascertaining the position to which the tool 5 is moved. Accordingly, it is extremely important to know exactly how far the position detection camera 11 is offset from the tool 5. However, the actual offset amount varies from instant to instant as a result of thermal expansion of the camera holding arm 18 and bonding arm 16 caused by radiant heat from the high-temperature bonding stage and heat transferred from the heated air. Accordingly, it is necessary to correct the offset amount when bonding work is initiated and at an appropriate timing during intervals between bonding operations.
Japanese Patent No. 2982000 (Japanese Patent Application Laid-Open (Kokai) No. 2000-021923), for instance, discloses a conventional means for determining an accurate offset amount. However, this offset amount determining means requires a special offset correction camera used for offset correction in addition to the position detection camera that detects the positions of bonding points. Thus, the construction is complicated and expensive.
Japanese Patent Application Laid-Open (Kokai) No. 2001-203234 discloses a system that solves the above problems. In this system, an optical means that conducts image light of a tool and a reference member disposed in a specified position to a position detection camera is employed. As a result, the position detection camera that detects a position of the workpiece is used also for imaging the tool and reference member. Thus, this prior art shows that there is no need to use a special offset correction camera even in cases where a reference member is used.
The above-identified Japanese Patent Application Laid-Open (Kokai) No. 2001-203234 discloses various embodiments. One of the embodiments uses a telecentric lens as the lens of the position detection camera. The xe2x80x9ctelecentric lensxe2x80x9d refers to a telecentric optical system or to an optical system which is constructed so that principal light rays that are focused as an image pass through the rear-side focal point of a lens. A telecentric lens has a broad tolerance range with respect to positional deviations in the direction that face the image focusing plane. Especially, even in a case in which transmitted light consisting of parallel light is illuminated, the size of the image (i.e., the distance from an optical axis) does not vary even if the object position fluctuates.
The optical system of the position detection camera in this prior art is similar to telecentric; strictly speaking, however, such an optical system is not telecentric; and if the object deviates from the focal position when the object is illuminated with parallel light, the image becomes more or less blurred.
The object of the present invention is to provide a bonding apparatus that can easily obtain a clear image regardless of the focal position when illumination is made with parallel light.
The above object is accomplished by a unique structure of the present invention for a bonding apparatus that comprises a bonding tool through which a wire passes and which performs bonding on a workpiece, a position detection camera which takes images of the workpiece, a reference member which is disposed in a specified position of the apparatus, and an optical means which conducts the image of the tool and reference member to the position detection camera; and in the present invention, an afocal system is employed which is constructed by a combination of a lens disposed in the position detection camera and a lens disposed in the optical means.
In the above structure, the afocal system is a Kepler type afocal system.
Instead, the afocal system can be a Galileo type afocal system.